Rigging issues and animation issues
The reason for the leg stretching is because the foot bone is controlling most of the leg mesh. The leg bones are only slightly affecting the mesh, and thus you get stretching when both bones are 'pulling' on the same mesh. You will need to properly weight paint your mesh (or manually assign them in edit mode) and then work on your skeleton. I would suggest looking up tutorials for setting up IK constraints as that should do what you want.